1. Field of the Invention
This invention relates generally to transmission and switching techniques in telephone communication systems and more particularly to an improved conference technique whereby a number of channels in a telephone switching system employing pulse code modulation for transmission purposes are combined so that a number of subscribers may participate in a common telephone conversation.
The present invention pertains to a conference circuit for use in a private automatic branch exchange similar to those units manufactured by GTE Automatic Electric Incorporated and designated GTD 120. Circuitry with minimum modification could also be employed in Class 5 central offices that employ digital switching. Such telephone systems employ a time switching network rather than the more prevalent earlier space divided switching network.
In time division switching networks a requirement exists to have sources of pulse code modulated voice samples associated with time slots. These time slots allow the conference to sequentially receive the code for each conferee. For the conference circuit to be effective it must be able to recognize who the conferees are and of course who is not associated with the conference. The circuitry must also be capable of distributing the conference speaker's code to each conferee. Information of this sort is of course available in the telephone switching systems referred to above. It should be understood that only telephone switching systems employing pulse code modulation can use the circuitry of the present invention and such circuitry interfaces with time division portions of such switching networks.
2. Description of the Prior Art
An approach to the handling of pulse code modulated information in conference circuitry is taught by U.S. Pat. No. 3,699,264 which is assigned to the same Assignee as the present invention. In the noted patent digital signals are not converted to analog, rather binary words are compared from the participating channels, with the largest binary numbers selected as the speaker. The reference patent utilizes a maximum binary code to select the speaker. A co-pending U.S. application, Ser. No. 709,580, filed July 28, 1976 by the inventors of this application and now U.S. Pat. No. 4,022,981 issued May 10, 1977, utilizes a minimum binary code as employed in the coding formats (D2 and D3) currently employed in pulse code modulated telephony.
PCM conferencing as taught in the above-identified patent and co-pending application, requires a source of pulse code modulated (PCM) coded voice samples which have associated time slots. These time slots allow the conference to sequentially receive a code for each conferee. The conference circuitry must be able to recognize who the conferees are and who is not associated with the conference call. The conference circuit then determines the loudest PCM voice sample during each PCM time frame, storing and outputting the selected PCM code to all conferees. This technique however for generating conferencing is subject to degradation due to the presence of idle channel noise and from non-talking conferees as the speakers audio signal passes through the region and its PCM sample is at a low weight value. In addition, when two or more conference members are conversing simultaneously the conference circuit could alternately select a new speaker for each time frame, thus severely clipping the speech of conversing conferees.